Google & Google suggest.
If you're looking for an alternative to Application X, just Google the phrases:
"Application X vs" and "vs Application X"
Googling "Dropbox vs" and "vs dropbox" shows the following alternatives on page 1:
SugarSync, Mozy, Wuala, Drop.io, ZumoDrive, box.net, JungleDisk, Live Mesh, Syncplicity.
Google suggest is even better because it instantly shows the items people are currently comparing.
Just type "Dropbox vs " into the google.com search box, and wait for the suggestions to appear.
Suggest gives the following "crowdsourced" suggestions for "dropbox vs":
sugarsync, mobileme, idisk, box.net, mozy, google docs, evernote, live mesh, zumodrive.